This invention relates to high speed marking devices, particularly for metal surfaces by impacted dot formation for permanent identification purposes.
It is becoming increasingly desirable to permanently mark different components in the course of manufacture. This is of particular importance in the automotive and aerospace industries, for example, where each component may require relevant markings. Marking devices can be fixed, portable, or hand-held.
For machine readable codes, precision of marking is of great importance. For other applications, precision is less important, and the main requirements may only be, for example, achieving a result readable by the naked eye. The fact that precision is of a reduced importance permits faster marking to be achieved, which is equally desirous, particularly in automated procedures. Furthermore, it is generally always desirous, particularly for handheld and portable devices, to have compact, light weight designs, which is also desired.
Marking heads that are driven by electrical solenoids are inherently slow. The time taken for a solenoid to operate to produce the marking action is unacceptably slow for high speed marking. Air-driven marking heads are used in preference, but they can also be slow when they are activated by electronically controlled valves. For higher speeds, such valves are not used. Instead, an air driven system is employed that causes the marking head to produce repetitive marking actions on a continuous basis whilst there is an operating air pressure present.
Stepper motors are commonly used to give position control for placement of the marking head within an operating envelope. They inherently provide low torque. To move the marking head at high speed generally requires torques that exceed those produced by stepper motors. Often the motor forms part of the components that the motor is attempting to move, in that the motor would also be moving itself. Thus making the motor larger compounds the problem of low torque. Moreover, the need for fast movement of the marking head increases the need for higher torque capacity.
Fast movement could be obtained through a suitable gearing ratio between the motor and the marking head, but this requires more torque capacity from the motor. Furthermore, this exacerbates a potential problem in the weight of the marking head overcoming the resistance in the system to fall to a low position when power is removed. It is desirable for this not to occur. Stepper motors have permanent magnets. The cogging torque provided by the magnets offers a degree of force that may prevent the rotor from turning when the power supply to the motors is not present. Depending on the position of the marking head, gravity may act on the marking head such that the cogging torque cannot prevent it from moving.
Some movable parts being heavy makes this worse. It is desired that, when power to the motor or motors is removed that the marking head retains its position until power is restored to the motor or motors. It is therefore desirable to have the motor separate to the moving components. This would not only reduce the load on the motor but would also reduce the weight of the movable parts.
U.S. Pat. No. 4,808,018 discloses a marking device comprising a pneumatically actuated, multiple pin marking head mounted on a carriage displaceable along an axis in a carriage frame that is pivotable about that axis in base frame, translation of the carriage in the carriage frame and pivoting of the carriage frame being effected by a single motor so that the pins of the marking head are progressively tracked over a target area, each pin being actuated when a mark is to be made on the target.
Such an arrangement is not suitable for fast marking because the entire field of a marking area is traversed, even when only selected pixels of the field are to be marked, and selective control of the marking pins must be provided.
EP-A-591092 (U.S. Pat. No. 5,316,397) discloses an arrangement similar to U.S. Pat. No. 4,808,018, but where drive to the marking head is provided by independent stepper motors operating through high helix angle lead screws. Nevertheless, solenoid actuated solenoid valves selectively fire the marking pins.
U.S. Pat. No. 5,368,400 discloses a pneumatically actuated single pin marking head mounted on a carriage displaceable along two orthogonal axes by two independent motors fixed in a base frame and connected by cables and a pulley system to the carriage.
U.S. Pat. No. 6,135,022 discloses a pneumatically actuated single pin marking head mounted on a carriage displaceable on a carriage frame by a belt drive from a first motor fixed in a base frame and pivotable about an axis by a second motor fixed in the base frame
It is an object of the present invention to provide a system which is fast but has reasonable accuracy, and which overcomes the aforementioned problems, or at least mitigates their effects.
In accordance with the present invention there is provided a marking device comprising:
a base frame;
a first motor fixed in the base frame;
a carriage frame pivoted in the base frame about a screw axis;
a marking head mounted on the carriage frame for movement thereon in a first direction parallel said screw axis, the marking head comprising a pneumatically driven, continuously reciprocating, marking pin;
a drive screw rotationally mounted in the frame along said screw axis and being driven by said first motor, the carriage frame being journalled on said drive screw to permit said pivoting about said screw axis, and the marking head being driven in said first direction by rotation of said screw; and
a second motor fixed in the frame, the carriage frame being pivoted by said second motor.
Preferably, a drive screw pulley is disposed on the drive screw, a belt transmitting drive from said first motor to the drive screw through said pulley. The drive screw pulley may be disposed between the carriage frame and base frame.
Preferably, the drive screw has a thread pitch of about 0.1 turns per mm and said pulley provides between 2:1 and 4:1 (preferably about 3:1) drive ratio torque reduction, whereby said motor makes about 0.03 turns per mm translational movement of the carriage in the carriage frame.
Preferably, the carriage frame comprises a bar on which the marking head slides and disposed parallel said drive screw. The marking head preferably comprises a body having a front end carrying said marking pin, and a rear end through which the marking head is connected to said bar, the drive screw passing through a nut in the body intermediate said ends.
Preferably, said bar has at each end thereof an arm pivoted on bearings on said drive screw. The marking device may further comprise two idler pulleys journalled on the base frame, a motor pulley driven by said second motor, and a belt disposed around said pulleys, and one of said arms may have an extension extending away from said drive screw and be fixed to said belt between said idler pulleys so that drive from the motor pivots the carriage frame about said drive screw.
Said extension may have a contact surface having a curvature centred on the drive screw, the contact surface having both radial and circumferential extent that is sufficient to approach contact with the belt around the idler pulleys, whereby the tension of the belt is substantially maintained during pivoting of the arm extension between the idler pulleys. Preferably, the belt is fixed to said tension surface centrally thereof.
Preferably, said extension arm is carriage frame at one end of said drive screw, and said drive screw pulley is at the other end of said drive screw. Preferably, both motors are disposed for rotation about axes parallel the drive screw and are offset with respect to one another and, with respect to the marking head, are disposed behind the carriage frame.
Preferably, the base frame comprises two plates, each mounting one motor and each mounting for rotation therein one end of the drive screw. In this event, a third base component links said frame plates rigidly together. Preferably, said third base component is a casing for the marking device in the form of a box section into which marking device is slid for final assembly.